This invention relates to a technique for copying patterns, used widely in the process of producing semiconductor devices, and more particularly to a light-reflection type pattern forming system using a light-reflection type mask instead of the conventional light-transmission type mask.
In the conventional technique for forming patterns, using a light-transmission type mask, a wafer comprising a silicon substrate and a film of silicon oxide formed thereon is used as an exposed object and the silicon oxide film is subjected to a photoetching process to cut a gap of desired pattern in the silicon oxide film. For this purpose, a photoresist layer is deposited on the silicon oxide film. In addition, an opaque material such an emulsion layer is deposited on a transparent substrate in a pattern similar to an original pattern. Light is then passed through the part of the transparent substrate not covered by the opaque pattern onto the photoresist layer and causes a photochemical reaction in the corresponding part of the photoresist film so that a desired pattern similar to the original one is copied on the exposed object. These are the preparatory steps for the etching which is to follow.
However, the conventional light-transmission type pattern copying technique described above has the following inherent drawbacks.
In general, a glass plate is used as the transparent substrate for the light-transmission type mask. However, the light used to cause a photochemical reaction or an exposing effect is necessarily absorbed by the transparent material. The degree of the absorption increases with the decrease in the wavelength of the exposing light. Accordingly, the range of the wavelength of available light which may be used for this process is limited to prevent too much absorption and moreover even if the light having a range of wavelengths exhibiting a high transmissivity is used for exposure, the exposure time required to form a desired pattern in the photoresist layer is too long owing to the loss of exposing light through absorption by the transparent material. For example, if a pattern copying process is carried out in which deep ultra-violet light, considered favorable for a micro-pattern, is projected through a light-transmission type mask using a transparent substrate of ordinary glass, almost the entire quantity of the projected light is absorbed and the light which pentrates the glass is almost nonexistent. As a result, it is not merely a matter of the above described photochemical reaction, i.e. exposure reaction, requiring too much time, but rather that the reaction never takes place. Even in the case where quartz glass or saphire is used for the transparent substrate, the transmittivity to the deep ultra-violet light is at best 50%.